Coatings and Linings over Concrete for Chemical Immersion and Containment Service – NACE SP0892
Surface Preparation Requirements
Physical and chemical damage should be repaired using procedures described in ICRI 310.1R
Other defects such as honeycombs, scaling, and spalling shall be patched prior to surface preparation
Surface voids, tieholes, bugholes, pinholes, and excess porosities should be filled prior to the application of a coating system
Protrusions such as form lines, fins, sharp edges, and spatter should be removed during surface preparation
Coating System Design Requirements
In addition, the coating system design shall address the following design details:
Joints and cracks (see Paragraphs A10, A11, and NACE Publication 6G19710)
Terminations
Ultraviolet (UV) light and weathering resistance (outdoor applications)
Skid resistance (flooring applications)
Appendix D: Coating System Properties
Fillers:
Second only to the resin system, the choice of reinforcing filler is critical to establishing the desired low permeability and mechanical properties required of the coating. Fillers reduce shrinkage and promote a coefficient of thermal expansion that more closely approximates that of concrete, thereby reducing stress at the interface of the two surfaces. Fillers are also selected to enhance handling and application properties. Here again, the choices are numerous, but selection can be limited to a few broad categories such as geometry, size, and chemical resistance.
Spherical or particulate fillers or aggregates are perhaps the best-known resin modifiers. Those in wide use are based on crushed silica or washed and dried silica sands. In addition to silica, particles based on carbon, aluminum oxide, zirconium oxide, silicon carbide, glass beads, or any of a number of naturally occurring corrosion-resistant minerals may lend unique properties to the composite.
Aside from the reinforcement and permeation resistance imparted by fillers, some materials provide special chemical resistance or electrical conductivity. For example, most silicates, including glass, can be chemically attacked by hydrofluoric acid (HF) and strong alkalis, whereas aluminum oxide has good resistance at operational temperatures lower than 160°F, and carbon fillers are virtually inert. Carbonaceous fillers are also useful in applications involving sodium hydroxide or strong oxidizing agents.
Because of their strong crystalline structures, aluminum oxide, silicon carbide, and zirconium oxide also find applications in abrasion-resistant composites.
Reinforced Coatings:
Fibers are known to be among the best reinforcing fillers. They occur naturally in many mineral forms or as the product or by-product of a synthetic operation. Corrosion-resistant glass fibers are available in several forms including woven fabrics, randomly oriented chopped-strand mat, or as independent short fibers. Because of their reinforcing quality, fiber selection plays a key role in enhancing the mechanical properties of the coating.
The inclusion of the chemically resistant woven fabric increases the strength of the composite, lowers the thermal expansion, and helps to distribute localized stresses due to minor crack movements in concrete substrates. Multiple-layer application also ensures the best resistance to pinholes.
Fabric-reinforced coating can be based on epoxy, polyester, or vinyl ester resin, filled with silica or carbon, and reinforced with either fiberglass or synthetic fabric. The reinforcement layer is normally placed between two layers of filled resin.
This type of coating normally consists of three layers: a trowel-applied filled base coat, a resin saturated reinforcement layer, and a trowel- or spray applied filled topcoat.
Another general coating design involves the use of chopped-strand mat 1.5 oz per ft2 as the primary reinforcement. This type of coating simulates the inner construction and chemical resistance of an all-fiberglass tank. Typical specifications include one or two layers of chopped strand mat, which can be sealed with a resin-saturated chemical-grade surface veil or pure resin.
A significant improvement in the performance of mat-reinforced coatings is the inclusion of a silica-filled resin basecoat. This basecoat contributes to a substantially lower permeation rate. It has the added contribution of enabling the mat to be easily supported on walls and ceilings. In addition, better continuity is achieved between the mat layers and the often-rough concrete surface, because mat layers are somewhat rigid and do not easily conform to an irregular contour.
Flake-Filled Coatings:
These composites are generally filled with 5 to 40 percent flakes by weight, depending on the size and aspect ratio of the flake. These coatings are either spray- or trowel-applied in at least two coats with a final thickness of 25 to 150 mil, depending on the type of service. Glass and mica are the most common flakes in use, although graphite also finds some applications requiring HF resistance or electrical conductivity.
Design, Installation, and Maintenance of Coating Systems for Concrete Used in Secondary Containment – SSPC-TU 2/NACE 6G197
Terminations
Where the coating does not cover the entire concrete structure, the design of the end line or edges of the coating are important to the performance of the coating. The concrete at the edges of the coating is often cut at an angle, with coating material applied into the angled concrete. This technique, called “keying in,” helps the coating resist the undercutting, peeling, or disbonding stress to which the system is subjected due to installation, thermal movement, and water penetration.
Penetrations
For secondary containment, a monolithic coating system is typically the preferred design. However, many containment areas also sup-port pumps, pipes, conduits, handrails, tanks, and other equipment. Attaching these items directly to the concrete substrate often causes penetrations or holes through the coating system. When possible, these penetrations are eliminated by alternative layout designs, such as using external pipe supports (see Figure 9). Remaining penetrations are sealed by various methods (see Figure 10).
Coating Thickness
The thickness of a coating system is usually determined by the requirements for its physical properties. Tensile and flexural strength, when expressed per unit of cross-sectional area, may be identical for a specific coating at two different thicknesses, but when expressed in terms of the applied system’s load-bearing capabilities, the thicker system will have a higher strength. Impact strength is also increased with thickness. For areas subjected to significant abrasion or gouging (e.g., from forklifts), increased thickness is used to extend the life of the coating.
Thickness affects chemical resistance only if the chemical permeates the coating, as with some solvents, or if the chemical etches the coating surface significantly. Permeability is inversely proportional to coating thick-ness. For coatings that are not significantly affected by the chemical to be contained, the thickness of the coating is determined solely by physical requirements. The thickness of coatings on concrete ranges from 10 mils for liquid coatings used in mild service to 250 mils or higher for highly filled or reinforced systems used in heavy traffic conditions. In addition to design requirements, minimum coating thickness for thin systems is determined by the surface profile to be covered, while cost is a consideration for thick systems. Thickness is also important where shrinkage of the coating is a factor. Coatings that use polymers subject to shrinkage during cure are usually applied in thin layers or with reinforcement to minimize shrinkage and the resulting stress. Solvent-based coatings are also applied in thin layers to allow the solvent to evaporate.
Ultraviolet Light and Weathering Resistance
All thermoset polymeric materials degrade when exposed to ultraviolet (UV) light. This degradation can cause loss of gloss, color changes, chalking, or catastrophic cracking. UV degradation can be compounded by other weathering factors such as rainwater, condensation, humidity, and thermal cycling. UV degradation in coatings can be reduced by the use of inorganic pigments and fillers, UV absorbers, and free-radical scavengers, as well as by the use of UV-resistant topcoats.
Skid Resistance
Where foot traffic is expected and where the floor of the containment area may be wet from spills, cleaning, or rainwater, skid resistance is an important safety factor. Skid resistance is accomplished by: (1) not top coating over the residual texture of a troweled or broadcast system, (2) brushing or rolling a texture (stipple) into a thixotropic topcoat, or (3) adding aggregate particles when applying a liquid-rich sealing topcoat.
Aesthetics
While appearance is not a functional requirement for secondary containment coatings, these systems are sometimes applied in locations that are highly visible and may need to be aesthetically pleasing. This factor is often not addressed in specifications or contracts. There are many reasons that coating systems may not look perfectly uniform. Mat-reinforced systems may have mat overlap lines or mat surface profile. Trowel-applied coatings may have trowel marks. Skid-resistant surfaces may have nonuniform texture. In addition, deteriorated concrete may not have been restored and substrate imperfections may remain.
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